Structural and solvent modulation of symmetry-breaking charge-transfer pathways in molecular triads

Whereas the photoinduced charge-transfer properties of electron donor-acceptor dyads are now well understood, those of symmetric conjugated architectures containing several identical donor-acceptor branches have started to be scrutinised much more recently. Here, we report on our investigation of the charge-transfer dynamics of a series of formally centrosymmetric triads consisting of a quadrupolar dihydropyrrolopyrrole core substituted with two identical diphenylethynyl lateral branches. Using a combination of time-resolved electronic and vibrational spectroscopies, we show that these molecules exhibit rich excited-state dynamics, which includes three different types of symmetry-breaking charge-transfer processes depending on the nature of the end substituents on the core and branches as well as on the solvent: (i) excited-state symmetry breaking within the core; (ii) charge transfer from the core to one of the two branches; (iii) charge transfer between the two branches. This investigation illustrates how the excited-state properties of symmetric conjugated molecules, including the nature and location of the exciton, can be controlled by fine tuning structural as well as environmental parameters.